1. Field of the Invention
The present invention relates generally to a forming method of a fine resist pattern, and more particularly to an improved forming method of a fine resist pattern with high precision.
2. Description of the Background Art
FIG. 1 is a schematic diagram showing the principle of an ARCOR (Anti Reflective Coating on Resist) method.
Referring to FIG. 1, an anti-reflection film 2 is formed on the upper surface of a positive-type photoresist 1. Air 3 is present over anti-reflection film 2. Where a refraction index of positive-type photoresist 1 is n.sub.R, a refraction index n.sub.A of anti-reflection film 2 is the square root of n.sub.R. A film thickness of anti-reflection film 2 is .lambda..sub.exp /4n.sub.A .times.n (.lambda..sub.exp is a wavelength of exposure light e.sub.1, and n is an odd integer). Where these requirements are established, reflected light e.sub.2 reflected at the interface of air 3 and anti-reflection film 2 to be directed to positive-type photoresist 1, and reflected light e.sub.3 reflected at the interface of anti-reflection film 2 and positive-type photoresist 1 to be directed to positive-type photoresist 1 mutually cancel, so that substantial reflection is eliminated.
With the ARCOR method, multiple reflection within a film, which may be a problem in usual photolithography using a single photoresist layer, is suppressed.
Specifically, referring to FIG. 2, a curved line (1) shows the relation between a resist film thickness and resist line width in the case of using a single photoresist layer. A curved line (2) shows the relation between a resist film thickness and resist line width in the case of employing the ARCOR method. As can be seen from the figure, in the ARCOR method, a change of the dimension of a resist pattern can be suppressed irrespective of a change of the resist film thickness, and in turn, a stable resist pattern can be obtained.
Although the ARCOR method is a superior method of suppressing multiple reflection within a film, as described above, undesired reduction of a resist may occur in the case of out of focus as shown in FIG. 3.
FIG. 3 (a) shows a cross sectional shape of a resist pattern in optimum focus, and FIG. 3 (b) shows a cross sectional shape of a resist pattern out of focus. Referring to FIG. 3 (b), undesired reduction occurs on an upper portion la of resist 1.
As shown in FIG. 4, defocus occurs, for example, when positive-type photoresist 1 applied on a substrate 4 with a step is exposed by a projection aligner. Specifically, when a focus is set on an A portion, a B portion is out of focus, whereby undesired reduction occurs on upper portion 1a of positive-type photoresist 1.
The problem of undesired reduction on a resist also occurs in a CEL technology (Contrast Enhanced Photolithography).
A conventional CEL technology will hereinafter be described briefly.
FIG. 5 shows schematic diagrams for the comparison of photolithography (b) to which the CEL technology is adapted, and usual photolithography (a) to which the CEL technology is not adapted.
Referring to FIG. 5 (a) (1), positive-type photoresist 1 is applied on substrate 4. Referring to FIG. 5 (a) (3), positive-type photoresist 1 is selectively irradiated with light using a mask 5. Referring to FIG. 5 (a) (4), positive-type photoresist 1 is developed. According to this method, as shown in FIG. 5 (a) (4), the problems of poor contrast and poor resolution arise. The CEL technology was developed for enhancement of resolution.
Referring to FIG. 5 (b) (1), positive-type photoresist 1 is applied on substrate 4. Referring to FIG. 5 (b) (2), a contrast enhancement layer 6 is applied on positive-type photoresist 1. As is also disclosed in Japanese Patent Laying-Open No. 2-212851, contrast enhancement layer 6 contains a material whose light transmission rate increases as it is exposed to exposure light (referred to as a color fading pigment component). More specifically, absorption of an exposure light wavelength, which is large before exposure, is gradually reduced as the exposure proceeds. Diazonium salt, stilbazolium salt, aryl nitroso salt and the like are known as color fading pigment components. Phenol-type resin is used as a coating forming component.
Referring to FIG. 5 (b) (3), positive-type photoresist 1 on which CEL layer 6 is applied is selectively irradiated using mask 5. Referring to FIG. 5 (b) (4), positive-type photoresist 1 is developed. According to the method, CEL layer 6 formed on positive-type photoresist 1 becomes substantially transparent at the exposed portion, in turn enhancing contrast between the exposed portion and an unexposed portion. As a result, a resist pattern of high resolution can be obtained.
In the CEL method described above, a resist pattern of high resolution can be formed. As shown in FIG. 6, however, the problem that a film thickness of resist 1 is reduced arises in the case of out of focus.
As described above, in the lithography using a conventional photoresist, the problem that a film thickness of a resist is reduced arises in the case of out of focus. In exposure by a projection aligner, the reduction due to the out-of-focus is especially significant. The reduction of the film thickness of the resist is caused by solubility of the surface of the resist in developer.
A LENOS (Latitude Enhancement Novel Single Layer Lithography) method is known as a method improving resistance of a surface of a resist to solution in developer (Journal of Photopolymer Science and Technology Vol. 2, No. 3, 1989). In the LENOS method, a resist is immersed in alkali liquid to make the surface of the resist insoluble.
In this method, however, a troublesome process to immerse a resist in alkali liquid is indispensable, which complicates a manufacturing process of a semiconductor device.